The goal of the planned research is to elucidate how the p53 tumor suppressor elicits apoptosis in tumor cells. Under some conditions, induction of p53 leads to its transactivation of a set of pro-apoptotic target genes. Nevertheless, a transcription-independent role for p53 in causing apoptosis has been documented. It is planned to examine both the transcription-dependent and -independent roles of p53 in apoptosis in three specific aims. First, we discovered that mutation of H 115 in the L1 loop of the p53 core domain produces p53 protein that is more effective than wild-type p53 in DNA binding, transactivation and cell cycle arrest and yet is impaired in apoptosis. We will examine the properties of ill 15N and other p53 variants mutated in the L1 loop region. The transcriptional program of L1 loop mutants and their effects in cells and in mice will be evaluated. Second, we will continue to explore in depth the apoptotic features of cells expressing a transcriptionally impaired mutant p53 (P53Q22/S23) in order to learn about the different pathways to cell death that p53 can bring about. Third, we will use a biochemical and proteomics approach to discover unique features of promoters of apoptotic gene targets with which p53 associates.